Post-Annealing Effects on the Microstructure and Magnetic Properties of Hot-Deformed Nd-Fe-B Magnets

Post-annealing effects on microstructure and magnetic properties of Nd-Fe-B die-upset magnets were investigated using the commercial melt-spun flakes with a composition of Nd
_13.6
Fe
_73.6
Ga
_0.6
Co
_6.6
B
_5.6
. The die-upset magnets from the melt-spun flakes were subjected to post-annealing at temperatures ranging from 400 °C to 800 °C for 1 h in vacuum and then again annealed at a constant temperature, 600 °C. The coercivity was largely enhanced about 2.4 kOe after the first post-annealing treatment at 600 °C. However, the coercivity decreased at temperatures except for 600 °C and 700 °C. In the case of high temperature, 800 °C, annealing caused the grain growth, resulting coercivity degradation. After the second post-annealing treatment at 600 °C, the coercivity of the magnets post-annealed at low temperatures below 500 °C increased, whereas the coercivity of the magnets post-annealed at above 700 °C decreased. The
$\alpha $
and
${N} _{\mathrm {eff}}$
values obtained from the result of temperature dependence of coercivity increased after post-annealing at 600 °C. These results indicate that post-annealing at 600 °C was effective in improving the grain surface structure, but could induce the local stray field in the magnet.